Ryan, a former EastEnders star, will take the role of wicked sorceress Nimueh for the new 13-part series, while Hurt will play the voice of the Great Dragon, the young Merlin's mythological mentor.

Santiago Cabrera, who played Isaac Mendez in the US drama Heroes, will appear as Lancelot, joining an all-star cast which includes Buffy the Vampire Slayer star Anthony Head and One Foot in the Grave's Richard Wilson.

The role of the sorceress will be Ryan's first part since Bionic Woman, which was dropped following disappointing ratings in the US. Ryan has also been signed up to star in ITV1's new romantic comedy Mr Eleven, although this is not expected to air until late autumn.

Julie Gardner, the head of drama at BBC Wales, which commissioned the programme, said Hurt would bring "real gravity" to the series.

The three main parts of the young Merlin, Arthur and Guinevere will be played by a trio of newcomers.

Colin Morgan, 22, who has appeared in Doctor Who and the Catherine Tate Christmas special as well as playing Esteban in the Old Vic production of All About My Mother last year, will take on the role of Merlin.

Bradley James, 23, who plays the young Arthur, has previously appeared in an episode of ITV's detective drama Lewis, while Angel Coulby, who plays Guinevere, has appeared in Doctor Who and ITV police drama Vincent.

Merlin, which is being made by production company Shine through BBC Wales, will be broadcast from September in the Saturday 7pm slot previously filled by Doctor Who and Robin Hood.

The drama is set in the mythical town of Camelot, and focuses on Merlin as a wizard on the brink of adulthood who clashes with a young Prince Arthur.

Gardner said: "In this new version, Merlin and Arthur are young contemporaries for the first time ever, bringing a much loved tale to a whole new generation with a fresh, youthful new look and approach for Saturday nights this autumn on BBC1."

Filming is currently underway in England, Wales and France.

The series has already been sold to a number of international broadcasters by distributor FremantleMedia Enterprises, including NBC in the US and CTV in Canada.

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